1. Field of the Invention
This invention relates in general to metal spray methods and apparatus and, in particular, to a plasma-arc spray method and apparatus employing a transferred arc between a non-consumable electrode and a consumable spray material. This invention is especially applicable to producing high quality plasma type thermal spray coatings when employing a wire as the consumable spray material.
2. Description of Prior Art
The process of thermal metal spraying has been performed for many years with various methods being used to melt materials and propel molten particles onto a substrate. Present metal spraying systems include fuel/oxygen systems, electric arc systems (non-plasma arc), and the non-transferred arc plasma system that melts powder in a hot gas stream and propels it to the substrate.
In the electric arc systems, two consumable feed wires are fed from a spray gun along intersecting paths. Current is applied to create an arc between the wires that melts the wires at their point of intersection. High velocity compressed air is discharged on the molten metal to produce atomized molten metal that is projected onto a workpiece. A primary problem with electric arc systems has been equipment failure and shut down upon the feed wires becoming shorted or welded together during metal spraying operations. In addition, these systems have been bulky and heavy which renders them difficult to use in confined spaces such as encountered in the shipbuilding and ship repair industries.
The plasma arc process known in welding, cutting and thermal spraying is a process in which heat is produced by a constricted arc between a non-consumable tungsten electrode and a workpiece (called a transferred arc), or between a non-consumable tungsten electrode and a constricted orifice (called a non-transferred arc). In the plasma arc process, a gas is ionized into a plasma state when it is passed through an arc which is established between two oppositely polarized electrodes. The plasma section of the arc is kept extremely hot by the resistance heating effect of the current passing through it.
Present plasma thermal spray systems are generally of the non-transferred arc type. The arc is established between a non-consumable tungsten electrode and a non-consumable body which contains an orifice through which the plasma leaves the region of the arc. A powder is added to the hot plasma gas stream as it leaves the orifice. This powder is melted and molten droplets are propelled onto a workpiece. The plasma powder spray process produces high quality spray coatings but requires the use of more expensive powder as the spray material and a complicated spray powder feed mechanism.
U.S. Pat. No. 2,982,845 by D. N. Yenni et al. and U.S. Pat. No. 4,370,538 by James A. Browning disclose metal spray systems in which a transferred arc is established between an electrode and a single spray wire. In these examples (in Browning see FIG. 5), flow of the ionizable gas from a gas source is established through the constricted orifice. A low current non-transferred pilot arc is established between the cathode electrode and the positively-polarized constricted orifice of the primary nozzle. The pilot arc heats and ionizes the primary gas into the plasma state, producing a plasma stream from the primary nozzle. The arc then transfers to the more positively-polarized spray wire through the conductive plasma stream. When the transferred arc has been established, the pilot arc may be interrupted. In Browning, the spray wire is disposed inside a diverging/converging inner bore which is disposed downstream from the constricted orifice. The diverging/converging inner bore is in turn disposed upstream of an exit bore. Thus the transferred arc process occurs within a first diverging/converging bore and the atomized metal spray produced thereby is directed through a second bore to the spray target. A secondary jet in the form of combustion products of an air/fuel burner is directed into the exit bore to accelerate the metal spray. In Yenni et al., the spray wire is also disposed at the upstream end of a confining chamber. In both of these designs, the confining chambers create the possibility of spray material clogging the nozzle and thus enhances the danger of clogging.